In cutting processing, grinding processing and the like of metallic materials, metal working oils have been heretofore used for the purpose of lubrication, cooling and the like between a metallic material to be processed and a processing tool rotating at a high speed. As metal working oils, water-insoluble metal working oils mainly composed of mineral oil and so on, and water-soluble metal working oils formed by diluting mineral oil, a surfactant and so on with water are known.
In recent years, water-soluble metal working oils have come into wide use because water-insoluble metal working oils have the disadvantage that they easily catch fire. The rotation speed of processing tools in cutting processing, grinding processing and the like of metallic materials have been increasingly enhanced for improvement of processing efficiency. Accordingly, shearing stress, frictional heat and the like that are applied to metal working oils have further increased. When large shearing stress or frictional heat is applied to a metal working oil, the metal working oil is partially micronized and thermally decomposed, so that the metal working oil is easily scattered around in the form of a mist. Particularly, water-soluble metal working oils generally have a lower viscosity, and thus may be more easily scattered in the form of a mist as compared to water-insoluble metal working oils.
When a mist of a metal working oil is scattered, a processing machine, a product and so on may be contaminated by the metal working oil. Operators may lose their health when a mist of a metal working oil is taken in the bodies of the operators through their respiratory organs. Under these conditions, it is desired to develop a water-soluble metal working oil which can be effectively inhibited from being scattered in the form of a mist when used for cutting processing, grinding processing and the like of metallic materials. For example, Patent Document 1 discloses a water-soluble metal working oil containing a polyalkylene oxide having an average molecular weight of more than 1,000,000, etc. in view of suppressing scattering of a mist.